The following studies are planned: (a) an investigation of the time-course and circadian pattern of negative and positive feedback effects of estradiol on LH, FSH and prolactin in the short-day and long-day female hamster; (b) an investigation of the negative and positive feedback effects of progesterone on LH, FSH and prolactin in the short-day and long-day female hamster; (c) a study of the facilitative effects of prolactin on ovarian function in the anestrous hamster; (d) investigation of the progressive changes in hormone secretion and ovarian function with spontaneous reproductive recovery; (e) a thorough study of the aggression of female hamsters toward conspecifics of both sexes as a function of seasonal changes in reproductive capacity; (f) more extensive work on the dampening of behavioral receptivity in steroid-primed short-day hamsters; and (g) studies of negative and positive feedback of estradiol and other steroids in the constant light-induced anovulatory rat.